Pulp and paper sludge (a byproduct of primary pulping operations, recycle streams or waste paper pulping and the like), as well as the products of its incineration, represent an environmental and disposal problem for manufacturers of pulp and paper. Generally, pulp and paper sludge is unsuitable for paper making, although it generally includes the same components--lignin, cellulose, hemicellulose, calcium carbonate, clay, and other inorganic components--as those present in the paper pulp itself.
Paper sludge has traditionally been disposed of by landfilling, composting, utilization by the cement industry, and by incineration. The latter option, in turn, creates another problem, namely, disposal of the resulting ash, which often constitutes up to 50% (and sometimes as much as 80% or higher) of the volume of the sludge itself. Calcium carbonate, in the form of precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC), typically constitutes 20% and up to 75% of dry sludge content. Calcium carbonate is a natural carbonate which is loaded, typically together with clay, into paper as a coating and filler to improve the mechanical characteristics as well as the appearance of paper. Despite their natural abundance, calcium salts are generally expensive products because of the difficulties and expenses of their purification from natural mineral deposits. For instance, paper-quality PCC is typically produced from natural limestone via many stages including the calcination of limestone in an industrial kiln (into either a calcitic or a dolomitic lime), slaking, slurrying, carbonating, and a number of refining steps.
Calcium-derived compounds undergo chemical changes when paper sludge is incinerated. These changes were outlined in recent articles (see Sohara, "Recycling Mineral Fillers from Deinking Sludges," Paper Recycling '96 Conf. 1996) (hereafter "Sohara"); Pera et al., "Paper Mill Sludge: A Source of Valuable Cement Additives," Paper Recycling '96 Conf. (1996). The organic components of sludge are completely destroyed during incineration. Thermal dehydration of clay results in calcined aluminosilicates, which form complex chemical compounds with decarboxylated calcium carbonate of general formula Ca.sub.n Al.sub.a Si.sub.b O.sub.c, that is, calcium aluminosilicates. Silica, which enters the thermally treated sludge from the fluidizing medium (sand) during the incineration process and also as a product of kaolin thermal breakdown, reacts with calcium oxide (derived from thermal decarboxylation of calcium carbonate) forming calcium silicate CaSiO.sub.3. Other minerals present in sludges (as pigments, fillers, traces of flocculants, etc.), such as those based on magnesium, potassium, titanium and others, make the composition of the mineral content even more complex. The particular species formed depends mainly upon the relative amount (and nature) of clay in the mineral fraction of the sludge, the amount of calcium carbonate, and the conditions of the thermal treatment.
Formation of calcium silicates in the course of incineration of lime-treated "green liquor" residues, containing calcium carbonate and silica, was described in Chattaraj et al., Indian Pulp & Paper, June-July 1981, at 21-28. Such an incineration leads to the formation of di- and tri-calcium silicates (presumably, larnite Ca.sub.2 SiO.sub.4 and gehlenite Ca.sub.2 Al.sub.2 SiO.sub.7 among them), which in turn form a fine gelatinous precipitate of calcium silicate dihydrate in the caustic liquor ("white liquor"). The authors reported that calcium silicates make calcination of calcium carbonate more difficult. Moreover, calcium silicate particles that are formed as a result of the calcination process are fused into irregularly shaped abrasive agglomerates unsuitable for paper filling and coating; see Johnston et al., Appita Journal, 49(6):397-402 (1996).
Unfortunately, the inorganic content of sludge and sludge-derived ash is generally largely or totally wasted. At best, the prior art describes utilization of incineration ash for production of low-end, impure products of limited market value. For example, Sohara details processing of such ash to precipitate calcium carbonate on the surface of the ash itself (without separation from the ash). In particular, an aqueous slurry of incineration ash is carbonated with carbon dioxide gas; calcium carbonate nucleates and grows during the precipitation reaction. The resulting mixture of precipitated calcium carbonate and ash still contains from 10% to 30% incineration ash, and represents an undifferentiated agglomeration of minerals and clay.
Lacking in the prior art is a cost-effective method of producing pure, high-grade calcium and other mineral salts from papermaking sludge or ash derived therefrom.